chapter, a circumstance which has made it one of the most interesting objects to observers. No motion has yet been certainly detected among the components.
SPECTROSCOPIC BINARY SYSTEMS.
Among the many striking results of recent astronomical research it would be difficult to name any more epoch-making than the discovery that great numbers of the stars have invisible dark bodies revolving round them of a mass comparable with their own. The existence of these revolving bodies is made known not only by their eclipsing the star, but by producing a periodic change in the radial motion of the star. How their motion is determined by means of the spectroscope has been briefly set forth in a former chapter. As a general rule the motion is uniform in the case of each star. We have described in a former chapter the periodic character of the radial motion of Algol, discovered by Vogel. This was followed by the discovery that α Virginis, though not variable, was affected by a similar inequality of the radial motion, having a period of four days and nineteen minutes. The velocity of the star in its apparent orbit is very great, about ninety-one kilometers, or fifty-six English miles, per second. It follows that the radius of the orbit is some three million miles. The mass of the invisible companion must, therefore, be very great.
Fig. 3. Radial Motion of a Binary System.
A new form of binary system was thus brought out which, from the method of discovery, was called the spectroscopic binary system. But there is really no line to be drawn between these and other binary systems. We have seen that as telescopic power is increased, closer and closer binary systems are constantly being formed. We naturally infer that there is no limit to the proximity of the pairs of stars of such systems and that innumerable stars may have satellites, planets or companion stars so close or so faint as to elude our powers of observation. Still, there is as yet a wide gap between the most rapidly moving visible binary system and the slowest spectroscopic one, which, however, will be filled by continued observation.
The actual orbit of such a system cannot be determined with the spectroscope, because only one component of the motion, that in the direction of the earth, can be observed. In the case of an orbit of which the plane was perpendicular to the line of sight from the earth
